The present invention relates to oximeters, and in particular to sigma-delta modulators used in connection with analog-to-digital conversion in pulse oximeters.
Pulse oximetry is typically used to measure various blood chemistry characteristics including, but not limited to, the blood-oxygen saturation of hemoglobin in arterial blood, the volume of individual blood pulsations supplying the tissue, and the rate of blood pulsations corresponding to each heartbeat of a patient. Measurement of these characteristics has been accomplished by use of a non-invasive sensor which scatters light through a portion of the patient's tissue where blood perfuses the tissue, and photoelectrically senses the absorption of light at various wavelengths in such tissue. The amount of light absorbed is then used to calculate the amount of blood constituent being measured.
The light scattered through the tissue is selected to be of one or more wavelengths that are absorbed by the blood in an amount representative of the amount of the blood constituent present in the blood. The amount of transmitted light scattered through the tissue will vary in accordance with the changing amount of blood constituent in the tissue and the related light absorption. For measuring blood oxygen level, such sensors have typically been provided with a light source that is adapted to generate light of at least two different wavelengths, and with photodetectors sensitive to both of those wavelengths, in accordance with known techniques for measuring blood oxygen saturation.
Known non-invasive sensors include devices that are secured to a portion of the body, such as a finger, an ear or the scalp. In animals and humans, the tissue of these body portions is perfused with blood and the tissue surface is readily accessible to the sensor.
Typically, the analog-to-digital conversion in a pulse oximeter is done using a sigma-delta modulator for analog-to-digital conversion after the signal is demodulated into the separate red and IR signals. An example of a pulse oximeter circuit using sigma-delta modulators is set forth in U.S. Pat. No. 5,921,921. This patent shows the use of two sigma-delta modulators, one for the red channel and one for the IR (infrared) channel. The sigma-delta modulators provide 1-bit of digital resolution, with the output of the sigma-delta modulator being filtered to produce a higher resolution signal. This is accomplished by using a fast oversampling rate (typically 1200 Hz) and then filtering to produce the slow, high resolution signal. The gain of the sigma-delta modulator in this patent is controlled by varying the width of the feedback pulse.